Hi, as I shoot film, after research, I found that all (?) of the solid ND filters have a cast. So how do you guys use ND filters? Do you just shoot early/late in the day and use slow film and maybe cheaper slower lenses that allows you to stop down to f/22 and maybe a weak ND filter and a CPOl perhaps?
Probably film effects, rather than from the spectrum of the filter. Your images demonstrate that the three colored emulsions on your film don't have the same reciprocity characteristics.
I just tend to overexpose by a stop, or a half stop if using slide. My knowledge with colour films is limited though as I only really use black & white.
I use slower film and if I have them with me ND filters. If the waterfalls fills most of the frame, start with 1/4 second or 1/2 second. As the waterfall becomes a smaller part of the photograph, I suggest that you lengthen the exposure time accordingly.
A different approach (and effect) that you might like to try out is multiple exposures of the waterfall on a single piece of film. For example instead of exposing at 2sec @ f 22 try 8 exposures at 1/4 sec at f22. Both Wynn Bullock and John Blakemore have used this technique to great effect. I was also very pleased with the resulting images when, many moons ago, I used to do in the field landscapes.